Activation Of Frontal And Parietal Regions May Be Involved In Conscious Perception

Among the most mysterious of brain processes are those underlying our conscious awareness of events in the world. Much research in this area has focused on the extent to which stimuli that are not consciously perceived can nonetheless be processed by the brain. By studying what is missing in brain activity associated with unconscious events, scientists hope to learn what is special about the brain activity that gives rise to conscious perception. In this issue (Nature Neuroscience, Vol. 4, No. 7, 01 July 2001), Stanislas Dehaene and colleagues take advantage of a technique known as masking to prevent words from being consciously perceived. For instance, a word flashed briefly on a computer screen is readily seen if presented in isolation, but if it is sandwiched between two frames of visual random noise that come before and after it, the word can be made imperceptible or masked - subjects deny seeing it even though it is present on the computer screen for just as long as when it is presented alone and perceived. Previous studies have shown that unconsciously perceived masked words can influence the processing of subsequently perceived words, for instance by speeding recognition if the subsequent word is the same, showing that the masked word was registered in the brain despite not being perceived. Thus far, however, inferences about how masked stimuli are unconsciously processed have been indirect.

Using functional magnetic resonance imaging, Dehaene and colleagues show that parts of the same brain networks activated by the conscious reading of words are also activated by masked words. Consistent with previous research on word recognition, unmasked (consciously perceived) words activate a succession of brain areas, starting with posterior regions and ending in the regions at the front of the brain. The striking finding of Dehaene and colleagues is that masked words also activate parts of this network. This activity, observed in the posterior and ventral areas of the word reading network, is greatly reduced from that evoked by consciously perceived words, but it is highly significant. In contrast, no activity was found in the frontal and parietal regions of the network. These results suggest that activation of frontal and parietal regions may be involved in conscious perception.